
The above result is analogous to the HQET Lagrangian, which is indeed a consequence of heavy-quark spin symmetry [1]}. We have refrained from performing a rescaling of the field \(V_v\)  by \(1/\sqrt{2m_V}\)  (which would remove the prefactor \(2m_V\) ) for the sake of this field retaining the canonical mass dimension of a vector field. The Feynman propagator for the vector field in HVET takes the form
\(\Pi _V^{\mu \nu }(k)= \frac{-i{\hspace{0.85358pt}}P_{\perp _v}^{\mu \nu }}{2m_V{\hspace{0.85358pt}}(v\cdot k+i\epsilon )} \,,\) 
